Installing blades in a wind turbine and wind turbine lifting systems

ABSTRACT

Methods for installing blades of a wind turbine are provided. The method comprises: providing a blade holder, wherein the blade holder comprises a connection element adapted to be attached to a mounting surfaces and a lifting equipment attachment, the blade holder being provided with a steering mechanism. Then, the blade holder is attached to the blade. The blade holder is hoisted with the blade towards the rotor hub with lifting equipment. The blade holder is attached to the mounting surface of the hub using the connection element. The rotor hub is rotated using the weight of the blade holder and the blade to a first desired position for mounting the blade to the rotor hub. The blade holder is detached from the hub. Then, the blade may be mounted to the rotor hub. A wind turbine blade holder for holding a blade and for use in lifting the blade is also provided.

The present disclosure relates to methods for installing a blade in awind turbine and wind turbine lifting systems.

BACKGROUND ART

Modern wind turbines are commonly used to supply electricity into theelectrical grid. Wind turbines of this kind generally comprise a towerand a rotor arranged on the tower. The rotor, which typically comprisesa hub and a plurality of blades, is set into rotation under theinfluence of the wind on the blades. Said rotation generates a torquethat is normally transmitted through a rotor shaft to a generator,either directly or through the use of a gearbox. This way, the generatorproduces electricity which can be supplied to the electrical grid.

During installation, one or more blades are to be mounted to the windturbine hub. The hub generally comprises a plurality of annular mountingflanges with openings. The blade can comprise a plurality of fasteners,such as bolts, or pins or studs at its blade root. During installation,these fasteners are to be fitted into the openings in the mountingflanges. The hub may additionally have further surfaces prepared forattachment of a component, lifting or manipulation of the hub.

The blades may be installed in a variety of ways, e.g. in asubstantially vertical manner or substantially horizontally or at otherinclined angles. Particularly, in offshore wind turbines, mounting ablade from above the hub may be very complicated, or at least require anenormous crane and jack-up barge. Such an installation method may thusbe very expensive and depending on the depth of the sea, blade and rotorsize may simply be impossible.

It is known to hoist a complete rotor assembly, i.e. the hub withblades, and mount it to e.g. the nacelle. But in order to mount acomplete rotor assembly, a large surface area is required, which istypically not available in the case of offshore wind turbines.

It is further known to mount an incomplete rotor assembly on thenacelle, e.g. the hub with two blades and subsequently, mount theremaining blade. In these cases, the rotor with the two blades isnormally mounted with the two blades pointing upwards, i.e. “bunny ears”configuration. There is thus no need for rotating the wind turbine rotoras the third blade could be vertically mounted from below. However, inorder to be able to perform these operations, the prevailing wind speedhas to be below a predetermined value for a prolonged period time. Theperiod of time depends on the expected length of the installation stepand a safety factor to be taken into account.

During certain seasons, windows of several hours in which the wind doesnot reach the predetermined value may not be available very often. Inpractice, this may mean that personnel and maintenance equipmentincluding e.g. expensive cranes and jack-up barges may be in stand-byduring days or even weeks. This can represent an enormous cost.

It is known to mount each of the plurality of blades substantiallyhorizontally or substantially vertically. This means that individualinstallation steps may require less time and may be performed at higherwinds, thus increasing the time windows available for installation.However, such methods require rotation of the hub between the mountingof one blade and another. In order to correctly position the hub, torqueis required for rotating the wind turbine rotor after mounting one bladein order to mount the next one. When not all blades have been mountedonto the hub, the hub is not balanced, so that the weight of one or moreblades has to be carried upwards when rotating the hub. Thecorresponding torque may thus be very high.

The torque may be delivered using the gearbox (when present) with anexternal power supply for rotating the rotor. Such a system may be usedin combination with a locking pin for maintaining the rotor in a desiredposition for installation. This is not possible in case of directlydriven wind turbines as no gearbox is provided between rotor andgenerator. Even though the generator may be suitable for operating as amotor, it will generally not be able to deliver the required torque torotate the hub, especially when the hub is unbalanced, i.e. when atleast one blade, but not all the blades have been mounted.

It is an object of the present disclosure to provide methods and systemsthat at least partially reduce one or more of the aforementioneddrawbacks leading to improved methods of installing wind turbine bladesespecially suitable for direct drive wind turbines and offshore directdrive wind turbines.

SUMMARY

In a first aspect, a method for installing a blade of a wind turbine isprovided. The wind turbine comprises a rotor hub positioned on a windturbine tower such that the rotor hub is configured to rotate around anaxis, the rotor hub comprising mounting surfaces. The method comprises:providing a blade holder for holding a blade, wherein the blade holdercomprises a connection element adapted to be attached to the mountingsurfaces and a lifting equipment attachment, the blade holder beingprovided with a first steering mechanism adapted to control anorientation of the blade holder with respect to the lifting equipmentattachment. The method further includes providing lifting equipment forlifting the blade holder. Then, the blade holder is attached to theblade. The lifting equipment is connected to the blade holder using thelifting equipment attachment. The blade holder is hoisted with the bladetowards the rotor hub with the lifting equipment. The blade holder canthen be attached to one mounting surface of the hub using the connectionelement. The rotor hub is then rotated using the weight of the bladeholder and the blade to a first desired position for mounting the bladeto the rotor hub. The blade holder can then be detached from the hub.Then, the blade may be mounted to the rotor hub.

It has been found that the use of the blade holder in order to rotatethe rotor during the installation of a blade in a wind turbine may be anadvantage. The weight of the blade holder and the blade may provide thetorque to rotate the rotor hub. In consequence, one or more blades maybe mounted to the rotor hub in a desired position e.g. horizontally,thus the need to reach positions for installing the blades higher thanthe hub may be avoided. Furthermore, the need of auxiliary tools e.g. awind turbine generator operated by a motor or an auxiliary gear motor torotate the rotor may be eliminated. In addition, the blades may bemounted one by one, at higher wind speeds. More time windows for suchinstallations may thus be available, thus shortening installation timeand reducing installation cost.

The blade holder may be provided with a connection element adapted to beattached to the mounting surfaces of the rotor hub. The connectionelement may be provided with a steering mechanism. In this way,different angular configurations of the blade holder may be achieved,and therefore the installation and the removal of the blade holder (andthus the blade) at different angles may be obtained.

According to one embodiment, a locking system for locking the hub oncethe first desired position for mounting the blade of the rotor hub isachieved may be provided. This may be performed by manually pushing apiston, a rod, a bolt or a similar element into a corresponding hole inthe rotor hub. Additionally or alternatively, some additional systemse.g. wind turbine generator operated as a motor or an auxiliary gearmotor may be operated for retaining the hub in a certain position.

In a second aspect, a wind turbine blade holder for holding a blade isprovided. The wind turbine blade holder comprises one connection elementadapted to be attached to a mounting surface of a wind turbine rotor huband a lifting equipment attachment.

In some examples, the blade holder may be provided with a first steeringmechanism for controlling an orientation of the blade holder withrespect to the lifting equipment attachment and/or a second steeringmechanism for controlling an orientation of the blade holder withrespect to the connection element to be connected to the hub.

The first and/or second steering mechanisms may comprise pneumatic, orhydraulic or electric actuators, e.g. a hydraulic piston. The firststeering mechanism may also be achieved by having the lifting equipmentattachment being displaceable along e.g. rail of the blade holder. Bychanging the position of the lifting equipment attachment with respectto the center of gravity of the blade holder (with blade), areorientation of the blade holder can be achieved.

This way, for example the second steering mechanism may perform therepositioning of the blade holder in a desired position e.g. horizontalduring the installation of a blade, thus the blade holder (and thus theblade) may apply the necessary to torque to the rotor hub. Inconsequence, the rotation of the rotor hub to a desired position may beachieved.

According to a still further aspect a wind turbine comprising a rotorhub, a plurality of blades mounted on the hub and a blade lifting systemsubstantially as hereinbefore described is provided.

BRIEF DESCRIPTION OF THE DRAWINGS

Non-limiting examples of the present disclosure will be described in thefollowing, with reference to the appended drawings, in which:

FIG. 1 is an illustration of a block diagram describing an example of amethod of installing a blade in a wind turbine;

FIGS. 2 a-2 h schematically illustrate a sequence of situations that mayoccur during the performance of a method for installing a first blade ofa wind turbine according to an example;

FIGS. 2 i-2 j schematically illustrate a sequence of situations that mayoccur during the performance of a method for installing a second bladeof a wind turbine according to an example;

FIGS. 2 k-2 s schematically illustrate a sequence of situations that mayoccur during the performance of a method for installing a third blade ofa wind turbine according to an example.

DETAILED DESCRIPTION

The FIG. 1 is an illustration of a block diagram describing an exampleof a method of installing a blade in a wind turbine. For the sake ofsimplicity, references to times indicated by an “imaginary” clock mayalso be included in descriptions relating to FIG. 1 (and furtherfigures). The references to times may be used to indicate approximatepositions of the blade holder and/or the blade holder and the blade byconsidering the blade holder or the blade holder and the blade as anhour hand of the “imaginary” clock.

Thus, for example, reference may be made to the blade holder or theblade holder and the blade pointing to 6 o'clock to indicate that theblade holder or the blade holder and the blade is vertically pointingdownwards.

At block 101, a blade holder may be provided. The blade holder may bee.g. a device for clamping a blade or a lifting beam with one or morecontrollable slings around the blade. The blade holder may be providedwith a first steering mechanism adapted to control an orientation of theblade holder with respect to the lifting equipment attachment. The firststeering mechanism may allow the installation and the removal of theblade at different angles e.g. one position between 9 o'clock and 7o'clock and between 3 o'clock and 5 o'clock. The actuation of thesteering mechanism may be operated by electric current or hydraulicfluid pressure or pneumatic pressure although other ways are possible aswell.

At block 102, lifting equipment may be provided such as e.g. a crane.The lifting equipment may also be provided with a steering mechanism. Atblock 103, the blade holder may be attached to the blade. The blade maycomprise a blade root portion e.g. a blade flange with fasteners foreventually mounting the blade to a rotor hub.

At block 104, the lifting equipment may further be connected to theblade holder. At block 105, the blade holder (and thus the blade) may behoisted towards the rotor hub. At block 106, the blade holder may beattached to one mounting surface of the hub using a connection element.Furthermore, the connection element may be provided with one or morefasteners e.g. studs or bolts. The fasteners may be situated at aproximal end of the connection element although some other positions maybe possible. The fasteners may allow a strong connection between theconnection element and the hub, and thus the torque to rotate the rotorhub may easily be borne.

At block 107, the rotor hub may be rotated using the weight of the bladeholder and the blade to a first desired position for mounting the bladeto the rotor hub. At block 108, the blade holder may be detached fromthe hub. Then, the blade may be mounted to the hub at block 109.

FIGS. 2 a-2 h schematically illustrate a sequence of situations that mayoccur during the performance of a method for installing a first blade ofa wind turbine according to an example. Same reference numbers denotethe same elements. The method is described below with reference to thesequences of situations illustrated by FIGS. 2 a-2 h.

In FIG. 2 a, the rotor hub 5 may be mounted on the nacelle (not shown)in such a way that the rotor hub 5 is rotatable around one axis withrespect to the nacelle. The rotor hub is shown comprising a firstmounting surface 23 adapted to receive the blade root, a second mountingsurface 22 adapted to receive the blade root and a third mountingsurface 21 adapted to receive the blade root. Each of these mountingsurfaces 23, 22, 21 may comprise a pitch system (not shown) for rotatinga blade to be mounted to the mounting surfaces. The rotation may beabout a pitch axis i.e. a longitudinal axis of the blade when mounted tothe hub.

The pitch system may be a “standard” pitch system, either hydraulic orelectromechanical. In some implementations, a more powerful pitch systemmay be provided.

The blade holder 3 of FIG. 1 is shown having a first steering mechanism1 and a second steering mechanism 2. The blade holder 3 may comprise alifting equipment attachment 24 configured to be attached to a liftingequipment (not shown). The blade holder 3 may be provided with aconnection element 4 e.g. studs or bolts configured to be attached toone mounting surface.

The first steering mechanism 1 e.g. a levelling system provided with ahydraulic cylinder may be located at the upper part of the blade holder3. The first steering mechanism may be attached between the liftingequipment attachment 24 and an upper portion of the blade holder 3. Thefirst steering mechanism 1 may be configured to rotate the blade holderat different angles with respect to a lifting equipment attachment 24.This way, the blade may be installed at different angles (with respectto a horizontal plane).

The second steering mechanism 2 may be e.g. a levelling system providedwith a hydraulic cylinder may be located at the lower part of the bladeholder 3. The second steering mechanism may be attached between theconnection element 4 and a lower support of the blade holder 3. Thesecond steering mechanism may be configured to rotate the blade holder 3with respect to the connection element 4. This way, once the bladeholder 3 may be attached to the rotor hub 5 using the connection element4, the blade holder 3 (and thus the blade 7) may be repositioned in ahorizontal position. Therefore, a torque may be induced to the rotor hub5 and the rotation of the rotor hub 5 may be achieved.

The hub may be configured to carry a plurality of blades. Only one blade7 is shown in FIG. 2 a.

The FIG. 2 a illustrates an initial situation. In this initialsituation, the rotor hub 5 may be situated in a position which may notbe appropriate for installing the first blade 7. Such a situation inwhich mounting surfaces 21, 22 and 23 are arranged so as to receiveblades in bunny ears configuration may typically result from lifting thehub towards the top of the tower or nacelle.

A way to define these positions is with reference to an angular positionof the hub. The 0° position for a blade may be regarded as a position inwhich it is arranged substantially vertically, pointing upwards. Themounting surfaces are thus arranged in the specific situation shown at60° (mounting surface 23), 180° (mounting surface (22) and 300° (or −60°for mounting surface 21).

The blade holder 3 may be attached to the first blade 7. The liftingequipment (not shown) may be attached to the blade holder 3 using thelifting equipment attachment 24. The blade holder 3 and the blade 7 maybe brought near the rotor hub using the lifting equipment. The bladeholder 3 (and thus the blade 7) may be situated in substantiallyhorizontal position using the first steering mechanism 1 e.g. alevelling system provided with a hydraulic cylinder.

In FIG. 2 b, the blade holder 3 connection element 4 is attached to thefirst blade mounting surface 23. The rotor hub 5 comprises a first blademounting surface 23. This way, the connection element 4 may be attachedto the first blade mounting surface 23 of the rotor hub. In thisparticular example, the connection element 4 is suited for beingconnected to the mounting surfaces of the blade. This way, there is noneed for extra mounting surfaces of any type at the rotor hub forholding the connection element 4.

In FIGS. 2 c and 2 d, the rotor hub 5 is rotated in a clockwisedirection using the weight of the blade holder 3 and the first blade 7to a desired position. The weight of the blade holder 3 and the blade 7induces a torque (due only to weight loads) in the rotor hub 5.Therefore, no additional systems e.g. a wind turbine generator operatedby a motor, an auxiliary gear motor are necessary for acting on the hub.Once the desired position for mounting the first blade 7 is achieved,the rotor hub 5 may be blocked using a locking mechanism e.g. anactuator for selectively switching the hub between an engaged positionfor rotatably fixing the hub to an axle and a disengaged position forfreeing rotation of the hub on the axle. The locking mechanism (notshown) of the hub may ensure that the hub 5 remains in the desiredposition during the entire mounting operation of a first blade 7.

In an alternative method, a first rotation of the hub (not carrying anyblades) may be carried out using a generator for the wind turbine, orsome auxiliary driving equipment.

In FIG. 2 e, the connection element 4 is detached from the first blademounting surface 23 of the rotor hub 5. The blade holder 3 (and thus theblade 7) may be separated from the rotor hub 5 using the liftingequipment (not shown).

In FIG. 2 f, the blade holder 3 (and thus first blade 7) is rotated to asubstantially horizontal position using the first steering mechanism 1e.g. a levelling system provided with a hydraulic cylinder. The firststeering mechanism 1 may be configured to install the first blade in anangle e.g. one position between 9 and 7 o'clock and between 3 and 5o'clock.

In FIG. 2 g, the blade holder 3 (and thus the first blade 7) is broughtinto contact with the rotor hub 5 using the lifting equipment (notshown). In this way, the blade root portion 8 of the first blade may beattached to the first blade mounting surface 23 of the rotor hub 5. Theblade 7 may thus be mounted horizontally. Mounting blades in asubstantially horizontal manner is generally a preferred option sincesuch an operation may be carried out at higher wind speeds than otheroperations, and importantly there is no need to lift a blade atsubstantial height above the hub. But other non-horizontal orientationsmay be possible depending on e.g. the particular configuration of thewind turbine.

The FIG. 2 h illustrates the first blade 7 mounted to the first blademounting surface 23 (and thus the rotor hub 5). The locking mechanism ofthe rotor hub may be disconnected. This way, once the first blade 7 ismounted, the rotor hub 5 may rotate in a clockwise direction e.g.towards a position in which the second blade mounting surface isarranged such as to receive a blade in a substantially horizontal manner(i.e. at 270°). At that point, the locking mechanism can be activatedagain.

Alternatively, the locking mechanism (not shown) of the rotor hub 5 maybe maintained activated, thus the rotor hub 5 may remain in a positionwherein mounting surface 23 is at the 90° position. The mounting surface22 is then at 210° position, which could also be appropriate to installthe second blade using a suitable blade holder. The blade holder 3 isdetached from the first blade 7. Therefore, the blade holder 3 may beready for the installation of a second blade.

Alternatively, the hub could be allowed to move and the lockingmechanism (not shown) of the rotor hub 5 could be reactivated e.g. whenthe rotor hub 5 is in a position wherein mounting surface 23 is at the120° position. The mounting surface 21 is then at 240° position, whichcould also be appropriate to install the second blade using a suitableblade holder. The blade holder 3 is detached from the first blade 7.Therefore, the blade holder 3 may be ready for the installation of asecond blade.

FIGS. 2 i-2 j schematically illustrate a sequence of situations that mayoccur during the performance of a method for installing a second bladeof a wind turbine according to an example. Prior to this, the hub may beunlocked in order to cancel the locking of the hub occurred in theprevious installation of the first blade. In FIGS. 2 i-2 j, sameelements denote the same numbers as described in FIGS. 2 a-2 h.

The FIG. 2 i illustrates a situation in which the rotor hub 5 may besituated in a position which may be appropriate for installing thesecond blade 10. The blade holder 3 is attached to the second blade 10.The lifting equipment (not shown) is attached to the blade holder 3using the lifting equipment attachment 24. The blade holder 3 and thesecond blade 10 may be brought near the rotor hub using the liftingequipment (not shown). The blade holder (and thus the second blade 10)may be situated at the 8 o'clock position using the first steeringmechanism e.g. a levelling system provided with a hydraulic cylinder.

The FIG. 2 j illustrates the second blade 10 mounted to the second blademounting surface 22. This way, once the second blade 10 is mounted, therotor hub 5 is substantially balanced.

FIGS. 2 k-2 s schematically illustrate a sequence of situations that mayoccur during the performance of a method for installing a third blade ofa wind turbine according to an example. Prior to this, the hub may beunlocked in order to cancel the locking of the hub occurred in theprevious installation of the second blade. In FIGS. 2 k-2 s, sameelements denote the same numbers as described in FIGS. 2 a-2 h.

The FIG. 2 k illustrates an initial situation for the installation ofthe third blade. In this initial situation, the rotor hub 5 may besituated in a position which may not be appropriate for installing thethird blade 15. In this situation, a crane or other lifting equipmentwould have to lift the blade to much higher heights in order to mountthe blade from above the hub.

According to this example, the blade holder 3 may be attached to thethird blade 15. The lifting equipment (not shown) may be attached to theblade holder 3 using a lifting equipment attachment 24. The blade holder3 and the blade 15 may be brought near the rotor hub 5 using the liftingequipment (not shown). The blade holder 3 (and thus the third blade 15)is situated in a substantially horizontal position using the firststeering mechanism e.g. a levelling system provided with a hydrauliccylinder.

In FIG. 2 l, the blade holder may comprise a connection element 4 isattached to the third blade mounting surface 21. This way, theconnection element 4 (and thus the blade holder 3 and the third blade15) is attached to the third blade mounting surface of the rotor hub 5.

In FIG. 2 m, the rotor hub 5 is rotated in a clockwise direction usingthe weight of the blade holder 3 and the third blade 15 to a firstintermediate position shown in FIG. 2 m. The first two blades weresubstantially in equilibrium, but the extra weight of the blade holder 3and the blade 15 induces a torque (due only to weight loads) in the hub.Therefore, no additional systems e.g. a wind turbine generator operatedby a motor, an auxiliary gear motor may be necessary for rotating therotor hub 5. Once the first intermediate position is achieved, the rotorhub may be blocked using the locking mechanism e.g. an actuator forselectively switching the hub between an engaged position for rotatablyfixing the hub to an axle and a disengaged position for freeing rotationof the hub.

In FIG. 2 n, the blade holder 3 (and thus the third blade 15) is rotatedto a position substantially horizontal using the second steeringmechanism 2. This way, the weight of the blade holder 3 and the thirdblade 15 again induces a torque (due only to weight loads) in the rotorhub 5.

In FIG. 2 o, as commented above in the FIG. 2 n, the extra weight of theblade holder 3 and the third blade 15 may again induce a torque (dueonly to weight loads) in the hub sufficient to overcome thecountertorque caused by the other blades. This way, the rotor hub 5 maybe rotated in a clockwise direction using the weight of the blade holder3 and the third blade 15 to a second intermediate position. Prior tothis, the hub may be unlocked to allow rotation of the hub.

In FIG. 2 p, the rotor hub 3 may again be blocked using the lockingmechanism. Once the rotor hub 5 is blocked, the blade holder 3 isrotated to a position substantially horizontal using the second steeringmechanism 2 e.g. a levelling system provided with a hydraulic cylinder.This way, the weight of the blade holder 3 and the third blade 15 againinduce a torque (due only to weight loads) in the hub.

By repeatedly returning the blade and blade holder upwards, e.g. to ahorizontal position, the torque delivered by the weight of the blade maybe increased, so that the hub can be rotated further again.

In FIG. 2 q, the rotor hub 5 may be rotated in a clockwise directionusing the weight of the blade holder 3 and the third blade 15 to adesired position. The weight of the blade holder 3 and the third blade15 induces a torque (due only to weight loads) in the hub. Therefore, noadditional systems e.g. a wind turbine generator operated by a motor, anauxiliary gear motor may be used for acting on the hub. Once the seconddesired position for mounting the third blade 15 is achieved, the rotorhub 5 again may be blocked using a locking mechanism e.g. an actuatorfor selectively switching the hub between an engaged position forrotatably fixing the hub to an axle and a disengaged position forfreeing rotation of the hub. The locking mechanism of the hub may ensurethat the hub remains in the second intermediate position.

In FIG. 2 r, the connection element 4 is detached from the third blademounting surface 21 of the rotor hub 5. The blade holder 3 (and thus thethird blade 15) may be separated from the rotor hub 5 using the liftingequipment. The blade holder 3 (and thus the third blade 15) is rotatedto a substantially horizontal position using the first steeringmechanism 1 e.g. a levelling system provided with a hydraulic cylinder.

In FIG. 2 s, the blade holder 3 (and thus the third blade 15) is broughtinto contact with the rotor hub using the lifting equipment. In thisway, the third blade root portion 16 of the third blade 15 may beattached to the third blade mounting surface 21 of the rotor hub. Theblade is mounted horizontally, although other non-horizontalorientations may be possible depending on, e.g., the particularconfiguration of the wind turbine.

A wind turbine comprising a rotor hub, a plurality of blades mounted onthe hub and a blade lifting system is provided.

Although only a number of examples have been disclosed herein, otheralternatives, modifications, uses and/or equivalents thereof arepossible. Furthermore, all possible combinations of the describedexamples are also covered. Thus, the scope of the present disclosureshould not be limited by particular examples, but should be determinedonly by a fair reading of the claims that follow.

1. A method for installing a blade of a wind turbine, the wind turbinecomprising a rotor hub positioned on a wind turbine tower such that therotor hub is configured to rotate around an axis, the rotor hubcomprising a mounting surface, the method comprising: providing a bladeholder for holding a blade, wherein the blade holder comprises aconnection element adapted to attach to the mounting surface and alifting equipment attachment, the blade holder being provided with afirst steering mechanism adapted to control an orientation of the bladeholder with respect to the lifting equipment attachment; providinglifting equipment for lifting the blade holder; attaching the bladeholder to the blade; connecting the lifting equipment to the bladeholder using the lifting equipment attachment; hoisting the blade holderwith the blade towards the rotor hub using the lifting equipment;attaching the blade holder to the mounting surface of the hub using theconnection element; using the weight of the blade holder and the blade,rotating the rotor hub to a first desired position for mounting theblade to the rotor hub; detaching the blade holder from the hub; andmounting the blade to the rotor hub.
 2. The method according to claim 1,further comprising using a locking system to prevent rotation of therotor hub after rotating the rotor hub to a first desired position formounting the blade to the rotor hub.
 3. The method according to claim 1,further comprising using the first steering mechanism to rotate theblade holder to a substantially horizontal position after detaching theblade holder from the hub.
 4. The method according to claim 1, whereinthe connection element is provided with one or more studs located at oneend of the connection element, the studs being configured to attach to amounting surface of the hub.
 5. A method for installing two blades to arotor hub of a wind turbine comprising the method according to claim 1and further comprising: attaching the blade holder to a second blade;hoisting the blade holder with the second blade towards the rotor hubusing the lifting equipment; and mounting the second blade to the rotorhub.
 6. The method according to claim 5, further comprising, afterhoisting the blade holder, rotating the blade holder to an angle ofbetween 0 and 60 degrees with respect to the horizontal plane using thefirst steering mechanism.
 7. A method for mounting a third blade to arotor hub of a wind turbine already carrying two previously mountedblades, comprising: providing a blade holder for holding the thirdblade, wherein the blade holder comprises one connection element adaptedto attach to a mounting surface of the rotor hub and one liftingequipment attachment, the blade holder being provided with a secondsteering mechanism adapted to control an orientation of the blade holderwith respect to the connection element; attaching the blade holder tothe third blade; hoisting the blade holder with the third blade towardsthe rotor hub using the lifting equipment; attaching the blade holder tothe mounting surface of the hub using the connection element; using theweight of the blade holder and the third blade, rotating the rotor hubto a first intermediate position for mounting the blade to the rotorhub; using a locking system for locking the hub, locking the rotor hubafter the hub has reached the first intermediate position; rotating theblade holder upwards using the second steering mechanism; releasing therotor hub from the locking mechanism to allow hub rotation; using themass of the blade holder and the third blade, rotating the rotor hubaround a hub axis to a second desired position for mounting the thirdblade to the rotor hub; detaching the blade holder; rotating the bladeholder to a substantially horizontal position using the first steeringmechanism; and mounting the third blade to the rotor hub.
 8. The methodaccording to claim 7, further comprising, after releasing the rotor hubfrom the locking mechanism: using the weight of the blade holder and thethird blade, rotating the rotor hub around its axis to a secondintermediate position for mounting the third blade to the rotor hub;using the locking system, locking the hub after the hub has reached thesecond intermediate position; rotating the blade holder upwards usingthe second steering mechanism; releasing the rotor hub from the lockingmechanism to allow hub rotation.
 9. The method according to claim 8further comprising a method for mounting two blades according to claim5.
 10. A wind turbine blade holder for holding a blade, comprising: aconnection element adapted to attach the blade holder to a mountingsurface of a wind turbine rotor hub; and a lifting equipment attachmentfor attaching the blade holder to lifting equipment.
 11. The windturbine blade holder according to claim 10, wherein the apparatuscomprises a first steering mechanism adapted to control an orientationof the blade, when held by the blade holder, with respect to the liftingequipment attachment.
 12. The wind turbine blade holder according toclaim 11, further comprising a second steering mechanism adapted tocontrol an orientation of the blade, when held by the blade holder, withrespect to the connection element.
 13. The wind turbine blade liftingsystem according to claim 12, wherein the connection element is providedwith one or more studs located at one end of the connection element, thestuds being configured to attach to a mounting surface of a wind turbinehub.
 14. The wind turbine blade holder according to claim 11, whereinthe first steering mechanism is configured to rotate the blade, whenheld by the blade holder, to an angle of between −30 and −60 degreesrelative to a horizontal plane.
 15. The wind turbine blade holderaccording to claim 14, further comprising a second steering mechanismadapted to control an orientation of the blade, when held by the bladeholder, with respect to the connection element.
 16. The wind turbineblade lifting system according to claim 15, wherein the connectionelement is provided with one or more studs located at one end of theconnection element, the studs being configured to attach to a mountingsurface of the hub.
 17. The wind turbine blade holder according to claim10, further comprising a second steering mechanism adapted to control anorientation of the blade, when held by the blade holder, with respect tothe connection element.
 18. The wind turbine blade lifting systemaccording to claim 17, wherein the connection element is provided withone or more studs located at one end of the connection element, thestuds being configured to attach to a mounting surface of a wind turbinehub.
 19. The wind turbine blade lifting system according to claim 10,wherein the connection element is provided with one or more studslocated at one end of the connection element, the studs being configuredto attach to a mounting surface of a wind turbine hub.
 20. The windturbine blade lifting system according to claim 10, wherein theconnection element is adapted to attach to a mounting surface of a windturbine hub.